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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    :root {
      --color: red;
    }

    .box {
      color: var(--color);
    }
  </style>
</head>
<body>
  <div class="box">
    我是一个盒子
  </div>
</body>
</html>